An organic piezoelectric material exhibiting piezoelectricity and pyroelectricity, which is usable for converting thermal or mechanical stimulation into electrical energy, has been conventionally known with respect to acoustic equipment such as a microphone or speaker diaphragm, measuring instruments such as various thermal sensors, a pressure sensor, an infrared detector and so forth, an ultrasound probe, a vibration sensor having a high sensitivity for detecting mutation of genes and proteins, and so forth.
As a piezoelectric and pyroelectric body, widely used is a so-called inorganic piezoelectric material, for example, a single crystal of quartz, LiNbO3, LiTaO3, KNbO3 or the like, a thin film made of ZnO, AlN or the like, and a sintered material composed of Pb(Zr, Ti)O3 system or the like, which have been subjected to a polarization. However, a piezoelectric material as such an inorganic material exhibits properties such as high elastic stiffness, high mechanical loss factor, high density, and high dielectric constant.
On the other hand, an organic piezoelectric material such as polyvinylidene fluoride (hereinafter, referred to as “PVDF”), polyvinylidene cyanide (hereinafter, referred to as “PVDCN”) or the like has been developed (refer to Patent Document 1, for example). The organic piezoelectric material has a feature in which capability of preparing a thin film accompanied with producing a large area and so forth is excellent, those in any shape and structure can be prepared, and low elastic modulus as well as low dielectric constant is realized, whereby high sensitivity detection is possible to be realized in consideration of application as a sensor. In contrast, the organic piezoelectric material was limited to the available temperature range since the piezoelectric and pyroelectric characteristic disappears at high temperature because of low heat resistance, and properties such as elastic stiffness and so forth are also largely reduced.
With respect to such a limitation, since a polyurea resin composition has a large dipole moment originated from a urea bond, and exhibits an excellent temperature characteristic as a resin, various studies for the organic piezoelectric material have been done. For example, disclosed is a so-called vapor deposition polymerization method by which a diisocyanate compound such as 4,4′-diphenylmethane diisocyanate (MDI) or the like and a diamine compound such as 4,4′-diaminodiphenylmethane (MDA) or the like are evaporated at the same time to form a polyurea film (refer to Patent Documents 2 and 3, for example). However, since as to a polyurea resin composition prepared by a vapor deposition polymerization method described herein, the molecular weight of the resulting oligomer or polymer becomes heterogeneous, the polyurea resin composition is formed in a state of insufficient orientation when it is polymerized with polarization. For this reason, a dipole moment originated from a urea bond can not be sufficiently utilized, and further improvement for the organic piezoelectric material has been demanded.
There is a report in which a ferroelectric liquid crystal compound is utilized as a piezoelectric material (refer to Patent Document 4, for example), but in the case of those which have been reported so far, since kinds of polarization groups in the liquid crystal molecule are limited, spontaneous polarization and piezoelectricity are not sufficient those satisfying performance desired as a piezoelectric material have not yet been found out.
Further, there is another report in which a composite piezoelectric material obtained by mixing a material exhibiting piezoelectricity in a base material made of a polymeric material is applied for an ultrasound transducer, but in the case of those which have been reported so far, an aggregate produced in a piezoelectric material caused discontinuous interface and thereby produced reflection and scattering of ultrasound waves, and the foregoing ultrasound transducer was unsuitable as an ultrasound transducer with which efficient transmission and reception are performed.